The short answer is that each and every team at Microsoft uses whatever fits their needs best. The fit depends on what type of project it is, what types of tests need to be written (proper unit tests, integration tests, functional tests, performance tests,
stress tests, etc.), as well as the experience of the people writing the tests.

Even within the ASP.NET team we have different projects using NUnit, xUnit.net, and MSTest all for managed code. And some JavaScript tests are written using qUnit. And we have yet other tests that are written using various custom test frameworks that aren't
available outside. We have yet other functional tests written using
LTAF, which was built internally, and is also available externally.